1. Field of the Invention
The present invention relates to an immersion exposure apparatus.
2. Description of the Related Art
A projection exposure apparatus for exposing a circuit pattern drawn on a reticle (mask) onto a wafer by a projection optical system has hitherto been used. Recently, an exposure apparatus having not only a higher resolution, but also more superior transfer accuracy and throughput has been demanded with an increasing interest. In particular, attention is focused on (liquid) immersion exposure as one approach for realizing a higher resolution. The immersion exposure is intended to further increase a numerical aperture (NA) of the projection optical system by using a liquid between the projection optical system and a wafer. The NA of the projection optical system is expressed by NA=n×sin θ (where n: refractive index of the liquid). Hence the NA can be increased depending on n by filling a gap between the projection optical system and a wafer with a liquid having a higher refractive index (n>1) than air. The larger NA reduces a limit value of resolution R(R=k1×(λ/NA)) (where k1: process constant and λ: wavelength of a light source) of the exposure apparatus.
Regarding the immersion exposure, there is known a local fill method for locally filling a liquid between a wafer and a final lens of a projection optical system (see a pamphlet of International Laid-Open No. 99/49504). In the local fill method, it is required that after the wafer has been immersed in the liquid, no liquids remain on the wafer. For example, when the liquid is water, the liquid remaining on the wafer causes a defect called a water mark. The water mark is presumably caused through such a mechanism that an impurity in an atmosphere or a component of a resist is dissolved in the remaining liquid and thereafter the water is evaporated. Also, if the liquid remaining on the wafer strikes against the liquid positioned under the projection optical system, bubbles are generated in the liquid. Because the bubbles diffusely reflect an exposure light, the amount of light exposure is reduced and the throughput is lowered. Further, the diffused reflection by the bubbles prevents the exposure light from reaching the wafer, and deteriorates transfer accuracy. When the liquid is, for example, water having large evaporation heat, the heat of the wafer is deprived with the evaporation of the remaining liquid, whereupon the water temperature is locally lowered. This results in deterioration of the transfer accuracy because the wafer is deformed depending on a temperature distribution.
Various methods have been proposed to solve the above-mentioned problem by adding a hydrophobic layer and a hydrophilic layer to a liquid holding member, or by employing a liquid holding member, which is variable in relative angle with respect to the substrate (see, for example, Japanese Patent Laid-Open No. 2005-150734 and No. 2005-286286).
In the exposure apparatus proposed by Japanese Patent Laid-Open No. 2005-150734, the liquid is held in place by a sealing member fixed to the projection optical system, and a hydrophobic layer and a hydrophilic layer are formed at the bottom of the sealing member. Stated another way, a leakage of the liquid is suppressed by setting a contact angle in a portion immersed with the liquid to be relatively small and a contact angle in a portion outside the immersed portion to be relatively large. Such an arrangement is effective when a stage is operated at a low speed. However, when the stage is moved at a high speed over a long distance, the liquid is excessively dragged with the wafer, thus resulting in a risk that the liquid may contact even the layer having the large contact angle. In order to return the liquid, which has been once brought into contact with the relevant layer, to the original position in a short time, the relevant layer is required to have a high water-sliding property. More specifically, a sliding angle has to be reduced. If a static contact angle is just high, the liquid cannot be returned to the original position in a short time. Therefore, the liquid is torn and remains on the wafer. The liquid remaining on the wafer gives rise to the above-mentioned problem.
Japanese Patent Laid-Open No. 2005-286286 discusses an exposure apparatus in which a sheet-like member, for example, a sheet made of polytetrafluoroethylene (PTFE), is disposed on the outer side of a nozzle member for supplying and recovering a liquid. A pressure variation of the immersion liquid is suppressed by adjusting a relative angle of the sheet-like member to a wafer depending on a contact angle of the wafer surface. To avoid the liquid from remaining on the wafer, however, it is useful that a surface having a large contact angle be arranged parallel to the wafer surface, as described in Japanese Patent Laid-Open No. 2005-150734. This is because a contact area between the surface having a large contact angle and the liquid is increased so as to effectively suppress a length by which the liquid is extended. If the sheet-like member is not parallel to the wafer surface, the liquid is more apt to remain on the wafer than the parallel arrangement. The liquid remaining on the wafer gives rise to the above-mentioned problem.